This invention relates to a method for controlling electroless plating baths such as electroless nickel plating baths and an apparatus used therefor, and more particularly, to a method and apparatus for controlling electroless plating baths so that stable plating can be continuously conducted for an extended period of time.
Electroless plating baths are susceptible to severe variation of their composition since a metal salt, a reducing agent and other ingredients are quickly consumed during plating. It is thus necessary to frequently complement such consumed ingredients to create a constant rate of deposition and uniform thickness and properties of deposits. For such purposes, a variety of methods and apparatus for the automatic control or replenishment of electroless plating baths have been proposed as disclosed in Japanese Patent Application Laid-Open Nos. 53-44434, 53-45631 and 54-8123.
However, electroless plating baths such as electroless nickel plating baths and electroless copper plating baths have a short effective life. As electroless plating is continued for an extended period of time, reaction by-products accumulate in the plating solution. The reaction by-products, which are believed to be decomposition products of a reducing agent and neutralization salts resulting from plating reaction, adversely affect such factors as plating rate and properties of deposits. With the accumulation of reaction by-products the replenishment of the ingredients consumed by plating cannot prevent reduction of the plating rate and variation of the composition and properties of the deposits. Accordingly, although an electroless plating solution is replenished either by the above-mentioned automatic or by manual replenishing methods, plating baths age or become unusable within a relatively short period of time. The rate of deposition is remarkably reduced and the properties of the resulting deposits vary considerably as compared with those of the initial plating solution, even when the metal ion and the reducing agent are replenished so as to increase their concentrations to the levels in the initial plating solution. Under such circumstances, the aged bath must be discarded and replaced by a fresh plating solution.
Even in the case where the above-mentioned automatic control of an electroless plating bath is employed to automatically replenish a plating solution, the degree of aging of the bath should always be separately monitored to accommodate such aging. Accordingly, plating baths must be discarded after a relatively short period of use in the prior art electroless plating processes combined with the above-mentioned automatic control method. The prior plating processes are also unsatisfactory with respect to bath maintenance.